Time-resolved transient optical and electron paramagnetic resonance spectroscopic studies of electron donor-acceptor thermally activated delayed fluorescence emitters based on naphthalimide-phenothiazine dyads.

The photophysics of naphthalimide (NI)-phenothiazine (PTZ) dyads were investigated as electron donor-acceptor (D-A) thermally activated delayed fluorescence (TADF) emitters. Femtosecond transient absorption (fs-TA) spectra show that the photophysical processes in non-polar solvents are in singlet localized state (LE, = 0.8 ps) → Franck-Condon singlet charge separation state (CS, = 7.8 ps) → CS state ( = 2.2 ns) → triplet state (LE, = 16 μs). The LE state is formed the spin-orbit charge transfer-intersystem crossing (SOCT-ISC) mechanism rather than the spin-orbit (SO)-ISC mechanism. In a polar solvent, the CS state has a much lower energy than the LE state; thus, the LE state is absent from the photophysical processes and no TADF was observed. Moreover, we found that the delayed fluorescence lifetime is related to the low-lying triplet state (LE or CS states). When the CS state is the low-lying triplet state, the TADF lifetime is shorter than that of the LE state as the low-lying triplet state. In the time-resolved electron paramagnetic resonance (TREPR) spectra, both LE (zero field splitting parameter = 2250 MHz, = -150 MHz) and CS ( = 430 MHz, = 0 MHz) states were observed. It is noteworthy that the electron spin polarization (ESP) phase pattern of the CS state was inverted at longer delay times as a consequence of the selective transition between the LE and CS states and a faster decay of one sublevel of the CS state. These results are strong and direct experimental evidence for the spin-vibronic coupling mechanism of TADF.